1. Technical Field of the Invention
The field of the invention is that of gravimeters using matter waves which allow gravity measurements to be obtained with a very high precision. The applications cover very broad technical fields going from oil prospection to navigation by terrain correlation.
2. Description of the Prior Art
Generally speaking, gravimeters based on matter waves use atoms in free fall separated by laser pulses of the “Raman” type.
Another possibility for gravimeters using matter waves consists in using atoms pre-cooled by laser to a very low temperature close to zero degrees Kelvin and trapped in the neighbourhood of a substrate or “atomic chip” throughout the detection cycle. An architecture of this type notably has the advantages of being very compact and of a reduced power consumption.
The principle of operation of this latter type of gravimeter consists in trapping the cold atoms in the neighbourhood of the chip by means of a magnetic field in a superposition of two internal states by means of a two-photon transition, also referred to as “π/2 pulse”, and in separating them into two separate packets of atomic waves by applying a microwave field which creates a different potential for the two internal states. The measurement axis coincides with the separation axis. This axis corresponds to the vertical axis in the case of a measurement of the local gravitational field g. Such a device is described in the Patent application FR 2 968 088 by the applicant and is entitled “Method and device for measurement of a local gravitation field, using matter waves integrated onto an atomic chip with microwave separation of the atoms”.
The phase-shift ΔΦg induced by the local gravitation field g is written:
                              ΔΦ          g                =                                                            MsT                s                            ℏ                        ⁢            g                    =          Kg                                    (                  Equation          ⁢                                          ⁢          1                )            
s being the separation distance between the two wave packets,
M being the mass of the atoms used,
Ts being the time during which the two wave packets are kept separated, and
ℏ the reduced Planck's constant.
The uncertainty in the measurement of g according to this principle notably depends on the uncertainty in the scale factor K. The stability of the latter is limited in particular by the stability of the distance s between the two wave packets during the measurement. Indeed, any fluctuation of the static DC or microwave MW magnetic field induces fluctuations of the distance s. The fluctuations in the magnetic fields are mainly due to the fluctuations in the current sources IDC and IMW flowing in the chip. With the current techniques, the latter can only be stabilized with difficulty to better than 10−5 in relative value.
The Patent application FR 2 968 088 describes a method allowing the influence of the fluctuations in the MW magnetic field over the distance s to be reduced to the order 2.
Another important point relates to the coherence time of the interferometer, which may be reduced under the effect of the involuntary fluctuations in the magnetostatic field. The technique generally used to render the system robust to these fluctuations consists in using a particular magnetic field and two internal states of the atoms of rubidium 87Rb, sometimes called “clock states” and which are as follows:
|1>≡|F=1, mF−1> and,
|2>≡|F=2, mF=1>
The magnetostatic field is centred on the point B0 approximately equal to 3.229 Gauss. In this configuration, the fluctuations of the DC magnetic field only act at the second order on the difference in energy between the two internal states in question. For this reason, this point of operation is sometimes referred to as “magic field”.
However, the effect of the application of the DC and MW magnetic fields simultaneously modifies the “magic field” condition, and to a greater extent the greater the MW field.